As a percentage of total units in NYC there are effectively no rent-able units withed from the market. The total vacancy rate is about 1.4% right now and and for anything under about $3k/mo it's sub 1%. There are probably a few thousand units being held to get buildings out from under rent control, but the city has 2.1 million or so renter occupied housing units. Even during the height of the pandemic when the well off were fleeing the city, vacancy topped out at ~4%.